Heat Transfer Plate Having Small Cavities For Taking Up A Thermal Transfer Material

ABSTRACT

A power semiconductor device module includes, among other parts, a DMB structure. The DMB structure includes a ceramic sheet, a top metal plate that is directly bonded to the top of the ceramic, and a bottom metal plate that is directly bonded to the bottom of the ceramic. A power semiconductor device die is attached to the top metal plate. The bottom surface of the bottom metal plate has a plurality small cavities. When the bottom metal plate is attached to another metal member, a material between the plate and the member (for example, thermal grease or a PCM or solder) is forced into the cavities. This results in an improvement in thermal transfer between the plate and the member. Such cavities can alternatively, or in addition, be included on a metal surface other than a DMB, such as the bottom surface of a baseplate of the module.

CROSS REFERENCE TO RELATED APPLICATION

This application is a continuation of U.S. Non-Provisional patentapplication Ser. No. 15/476,976, filed Apr. 1, 2017, of whichapplication is incorporated by reference herein in its entirety.

TECHNICAL FIELD

The described embodiments relate to the joining a metal member toanother metal member so that there is good thermal transfer between thetwo members, and more particularly relate to DMB structures in powersemiconductor device modules.

BACKGROUND INFORMATION

A power semiconductor device module includes an injection molded plastichousing. Within the housing is a DMB (Direct Metal Bonded)/semiconductordevice assembly. The DMB/semiconductor device assembly includes a DMBstructure. The DMB structure can either be a so-called DCB (DirectCopper Bonded) substrate or a so-called DAB (Direct Aluminum Bonded)substrate. In either case, the DMB structure is a multi-layer structurethat includes an insulative but heat conductive center ceramic substratelayer. A planar bottom direct metal bonded metal layer is bonded to thebottom surface of the ceramic layer, and a planar top direct metalbonded metal layer is bonded to the top surface of the ceramic layer. Inthe case of the DMB being a DCB, these top and bottom metal layers arecopper or a multi-layer structure involving copper. In the case of theDMB being a DAB, these top and bottom metal layers are aluminum or amulti-layer metal structure involving aluminum. The top metal layer istypically patterned into a plurality of islands of metal. Discretesemiconductor device dice and possibly other electronic parts are thensurface mounted to the various islands on the top the DMB structure. Thevarious discrete semiconductor device dice and various portions of theDMB structure are then typically interconnected by bonding wires, bothto each other, as well as to external connection terminals of themodule, such that the overall wire bonded assembly is a desired powerdevice circuit.

In one type of module, the bottom metal layer of the DMB forms a part ofthe outside surface of the module. This bottom metal layer of the DMB issupposed to be coupled to a heatsink. There is generally thermal greaseor a phase change material disposed between the bottom of the DMB andthe heatsink. Heat generated by the semiconductor device dice and othercomponents within the module passes from the DMB, in some measurethrough the thermal grease or phase change material, and to theheatsink. An example of this type of module is the Y2-DCB module typeavailable from IXYS Corporation, 1590 Buckeye Drive, Milpitas, Calif.

In a second type of module, the module includes a metal baseplate. Thebottom metal layer of the DMB is mounted to the top of the metalbaseplate. The bottom of the DMB does not form an outside surface of themodule, but rather the bottom of the metal baseplate forms the outsidesurface of the module. The module is then mounted to a heatsink byplacing an amount of thermal grease or a phase change material betweenthe bottom of the metal baseplate and the heatsink. The module ispressed down onto the heatsink and is held in place by screws or bolts.Within the module, the DMB is either soldered to the top of the metalbaseplate, or is pressed against the top of the metal baseplate withthermal grease or a phase change material disposed between the DMB andthe metal baseplate. Heat generated by the semiconductor device dice andother components within the module passes down through the DMB, throughthe metal baseplate, and in some measure through the thermal grease orphase change material, and to the heatsink. An example of this type ofmodule is the Y1-Cu baseplate module type available from IXYSCorporation, 1590 Buckeye Drive, Milpitas, Calif.

SUMMARY

A power semiconductor device module includes, among other parts, a DMB(Direct Metal Bonded) structure. The DMB structure includes aninsulative ceramic sheet member, a top metal plate that is directlybonded to a top surface of the insulative ceramic sheet member, and abottom metal plate that is directly bonded to a bottom surface of theinsulative ceramic sheet member. A power semiconductor device die isattached to the top metal plate. The bottom surface of the bottom metalplate has a plurality small cavities. In one example, these cavitiesform a two-dimensional array of cavities. An “A by A square area” of thebottom metal plate includes at least a part of each of B of thecavities. Each of the B cavities of the A by A square area has a minimuminside width dimension that is not more than C millimeters across. Eachof the B cavities has a depth that is less than D millimeters. The A byA square area of the bottom metal plate has an aggregate cavity volumeof between E cubic millimeters and F cubic millimeters. In one example,A is 10.0 mm (ten millimeters); B is ten; C is 1.0 mm; D is 0.5 mm; E is0.001 mm³ (0.001 cubic millimeters); and F is 10.0 mm³.

In a first case, the module includes a metal baseplate. The DMBstructure and the metal baseplate are pressed together and are heldtogether under force, with an amount of either thermal grease or a phasechange material between. Due to the cavities, some of the thermal greaseor phase change material is forced up into the cavities. This improvesthe thermal transfer between the DMB structure and the metal baseplate.Alternatively, rather than there being thermal grease or a phase changematerial between the DMB structure and the metal baseplate, the bottommetal plate of the DMB structure may be directly soldered to the topsurface of the metal baseplate. In this case, an amount of solder thatis liquid may be forced up into the cavities during the solderingprocess.

In a second case, the module has no metal baseplate. Rather, the bottomsurface of the bottom metal plate of the DMB structure is an outsidesurface of the module. The module may be attached to a planar metalsurface of a heatsink by pressing the bottom surface of the bottom metalplate of the DMB structure against the planar metal surface of theheatsink, with an amount of thermal grease or a phase change materialbetween. Alternatively, the bottom metal plate of the DMB structure canbe soldered directly to the heatsink.

In third case, the module has a metal baseplate but it is the bottomsurface of the metal baseplate that has the array of cavities. When themodule is mounted to a heatsink, the bottom surface of the metalbaseplate is pressed against a planar metal surface of the heatsink andis held in place under force, with an amount of thermal grease or aphase change material between. Alternatively, the bottom surface of themetal baseplate can be soldered directly to the heatsink.

In any of these three cases, the attachment of the metal surface thathas the cavities can be performed under reduced pressure (less thanatmospheric pressure) so that air in various parts of the assembly isextracted.

The notion of a metal member having a planar surface with cavities asdescribed above is not limited to use in power semiconductor devicemodules, but rather is generally usable where a metal member is to bejoined to another metal member with good thermal transfer between thetwo members.

Further details, embodiments, techniques and methods are described inthe detailed description below. This summary does not purport to definethe invention. The invention is defined by the claims.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings, where like numerals indicate like components,illustrate embodiments of the invention.

FIG. 1 is a top-down perspective diagram of a power semiconductor devicemodule 1 in accordance with one novel aspect.

FIG. 2 is a top-down perspective diagram of module 1, but with its tophousing portions 4 and 5 removed.

FIG. 3A is an exploded view of module 1, seen in perspective.

FIG. 3B is an exploded view of module 1, seen in cross-section.

FIG. 4 is a cross-sectional side view of the DMB/semiconductor deviceassembly 15 within module 1.

FIG. 5 is a perspective diagram of the bottom of module 1.

FIG. 6 is an exploded perspective diagram of the bottom of module 1, butwith housing portions 4 and 5 removed.

FIG. 7 is a diagram of the bottom of the DMB structure of module 1.

FIG. 8 is a diagram like that of FIG. 7, except that an “A by A squarearea” 52 of the bottom metal plate 36 is indicated by the dashed squareline.

FIG. 9 is an expanded view of the detail area 53 of FIG. 8.

FIG. 10 is an expanded cross-sectional side view of the detail area 53of FIG. 9.

FIG. 11 is a table that sets forth dimensions and parameters for module1.

FIG. 12 is view of the detail area 53 of FIG. 8, but in a thirdembodiment in which the cavities do not extend all the way through thebottom metal plate.

FIG. 13 is a cross-sectional side view of the detail area 53 of FIG. 12.

FIG. 14 is a diagram of the bottom of the DMB structure in accordancewith a fourth embodiment.

FIG. 15 is a diagram of the bottom of the DMB structure of module 1 inaccordance with a fifth embodiment.

DETAILED DESCRIPTION

Reference will now be made in detail to some embodiments of theinvention, examples of which are illustrated in the accompanyingdrawings. In the description and claims below, when a first object isreferred to as being disposed “over” or “on” or “onto” a second object,it is to be understood that the first object can be directly on thesecond object, or an intervening object may be present between the firstand second objects. Similarly, terms such as “upper”, “top”, “up”,“down”, “downward”, “upward”, “vertically”, “horizontally”, “laterally”,“lower”, “under”, “below” and “bottom” are used herein to describerelative orientations between different parts of the structure beingdescribed, and it is to be understood that the overall structure beingdescribed can actually be oriented in any way in three-dimensionalspace.

FIG. 1 is a top-down perspective diagram of a power semiconductor devicemodule 1 in accordance with one novel aspect.

FIG. 2 is a top-down perspective diagram of module 1, but with its tophousing portions 4 and 5 removed.

FIG. 3A is an exploded view of module 1, seen in perspective.

FIG. 3B is an exploded view of module 1, seen from the side incross-section. The cross-sections of FIG. 3B are taken along sectionalline H-H′ of FIG. 3A.

FIG. 4 is a cross-sectional side view of the DMB/semiconductor deviceassembly 15 within module 1.

FIG. 5 is a perspective diagram of the bottom of module 1.

FIG. 6 is an exploded perspective diagram of the bottom of module 1, sothat the bottom surface of the bottom metal plate 36 of theDMB/semiconductor device assembly 15 can be seen. The housing portions 4and 5 of the overall module housing are not present in the diagram.

The module 1 includes a metal baseplate 2, an injection moldedinsulative plastic housing involving three portions 3, 4, and 5, twopress-fit pins 6 and 7, three large external connection terminals 8-10,four smaller external connection terminals 11-14, a DMB/semiconductordevice assembly 15, wires that connect the DMB/semiconductor deviceassembly to the three large external terminals (one of these wires 17 isillustrated), four screws 22-25 that hold the baseplate to the housing,an amount of soft silicone gel (not illustrated), an amount of epoxyencapsulant (not illustrated), and an amount of thermal grease 27. Threebolts 28-30 extend through corresponding holes in the three largeexternal connection terminals 8-10 and engage threads on threecorresponding nuts. (Although in the particular example module shown,the four screws 22-25 extend in from the bottom of the baseplate, inother example modules the screws extend in the other direction fromhousing portion 3, and through housing portion 3, and thread intothreaded holes in the baseplate.)

The DMB/semiconductor device assembly 15 includes a DMB structure 31 andplurality of bare semiconductor device dice 32-34. In the presentexample, each of the bare semiconductor device dice is a separatediscrete power semiconductor device die such as a discrete powertransistor (IGBT or power MOSFET), a discrete power diode, or a discretepower thyristor. A bare semiconductor die can also involve a combinationof these discrete devices. The dice are soldered to the top plates 37A,37B and 37C of the DMB structure. In addition to these baresemiconductor device dice, the DMB/semiconductor device assembly 15 canalso include one or more other components such as a resistor, acapacitor, or an inductor.

The DMB structure 31 of the DMB/semiconductor device assembly 15 caneither be a DCB (Direct Copper Bonded) substrate or a DAB (DirectAluminum Bonded) substrate. In either case, the DMB structure 31 is amulti-layer structure that includes an insulative but thermallyconductive center ceramic substrate layer 35, a planar bottom directmetal bonded metal layer 36 that is bonded to the bottom surface of theceramic substrate layer, and a planar top direct metal bonded metallayer 37 that is bonded to the top surface of the ceramic substratelayer. The ceramic substrate layer 35 may be, for example, alumina(Al₂O₃) or aluminum nitride (AlN). The top metal layer 37 is patternedinto the plurality of islands 37A, 37B and 37C of metal. Each of themetal islands 37A-37C and the bottom metal layer 36 is also referred toas a “plate”. For additional details and information on DMB structures,and how to make a DMB structure such as DMB structure 31, see: U.S. Pat.Nos. 6,404,065, 6,798,060, 7,005,734, and 9,111,782 (the entire subjectmatter of each of these four patent documents is incorporated herein byreference). The dice 32-34 are surface mount soldered to the islands37A-37C of the top metal layer 37 of DMB structure 31. The variousdiscrete semiconductor device dice and various islands of the DMBstructure 31 are electrically interconnected by heavy aluminum bondingwires or clips. One of these wires, wire 38, is illustrated. Other heavywires provide electrical connections between the DMB structure 31 andthe external connection terminals 11-14 of the module. One of thesewires, wire 17, is illustrated. In the case of the DMB being a DCB, thetop and bottom metal layers 37 and 36 are copper or a multi-layerstructure involving copper. In the case of the DMB being a DAB, the topand bottom metal layers 37 and 36 are aluminum or a multi-layer metalstructure involving aluminum. In the example illustrated here, the DMBstructure 31 is a DCB.

As shown in FIG. 6, the DMB/semiconductor device assembly 15 is glued tothe bottom of the housing portion 3. After wires (bonding wires andwires that couple to external terminals) are provided, the soft siliconegel (not shown) is placed over the electronics in the housing so thatthe wires and dice are covered. The silicone gel is allowed to cure. Anamount of epoxy encapsulant (cured also, not shown) is placed over thesilicone gel so that the bottom of the central cavity of the firsthousing portion 3 is substantially filled. On the bottom of theDMB/semiconductor device assembly 15, the amount of thermal grease 27 isapplied. The metal baseplate 2 is pressed up against the bottom of theDMB/semiconductor device assembly 15. The four screws 22-25 are thenthreaded into engaging threaded holes 39-42 in the baseplate 2, andthread farther up into threaded holes 43-46 in the housing portion 3.When the screws 22-25 are tightened, the metal baseplate 2 and thehousing portion 3 are drawn together, thereby forcing the top surface ofthe metal baseplate 2 into good thermal contact with the bottom surfaceof the bottom plate 36 of the DMB/semiconductor device assembly 15. Thethermal grease 27 is pressed between the bottom metal plate 36 of theDMB structure 31 and the top of the metal baseplate 2.

As shown in FIG. 1, the metal baseplate 2 has four mounting holes 47-50.Four corresponding bolts (not shown) are fitted into these four holes47-50 and are torqued down in order to attach the bottom of the metalbaseplate 2 to the top of a heatsink (not shown). Another amount ofthermal grease or a phase change material (not shown) is disposedbetween the bottom of the metal baseplate 2 and the top of the heatsink(not shown) so that there is good thermal contact between baseplate 2and the heatsink (not shown).

FIG. 7 is a diagram of the bottom of the DMB structure. Both the bottommetal plate 36 and the ceramic layer 35 are both octagonally shaped asillustrated, but the bottom metal plate 36 is slightly smaller than theceramic layer 35. Each small dot of the two-dimensional array of smalldots in the illustration represents one cavity. Each of the cavitiesextends into the bottom metal plate 36 from the plane of the bottomsurface 51 of the bottom metal plate 36.

FIG. 8 is a diagram like the diagram of FIG. 7, except that an “A by Asquare area” 52 of the bottom metal plate 36 is indicated by the dashedsquare line. The “A by A square area” 52 has four sides of equal lengthA. In addition to this “A by A square area” 52, a detail area 53 is alsoindicated. There are two cavities 54 and 55 in this detail area 53.Other than for the cavities illustrated in FIG. 8 on the bottom surfaceof the bottom metal plate 36, the entire remainder of the bottom surfaceof the bottom metal plate 36 is substantially planar.

FIG. 9 is an expanded view of the detail area 53 of FIG. 8. When viewedfrom the perspective of looking at the DMB structure 31 straight on asin the view of FIG. 8, each cavity has a circular shape. The circle isof a diameter of C. Diameter C is also the minimum inside width of eachof the individual cavities in the “A by A square area” 53.

FIG. 10 is an expanded view of the detail area 53 of FIG. 8, but in thecase of FIG. 10 the view is a cross-sectional side view taken alongsectional line G-G′ of FIG. 9. Each of the cavities 54 and 55 is of thesame depth, and this depth does not exceed dimension D. The number andsize of the cavities in the “A by A square area” 52 is such that theaggregate cavity volume is in a range between a bottom range value of Eand a top range value of F. In the present example, the 0.30 millimeterthickness of the bottom metal plate 36 is the same as the thickness ofthe top metal plate 37. The center ceramic layer 35 is alumina and is0.63 millimeters thick. The two plates 36 and 37 are both patterned witha protective photoresist, and are then simultaneously etched. Afteretching, the photoresist is stripped. In one example, the two plates 36and 37 are etched by placing the DMB structure 31 into a wet etchingsolution, so the islands 37A-37C are formed out of the top metal plate37 at the same time that the bottom metal plate 36 and thetwo-dimensional array of cavities is formed. Because both the top plateand the bottom plate of the unpatterned DMB are etched at the same time,there is very little if any additional cost to providing the novelcavities in the bottom metal plate 36 as compared to a prior art methodof making the DMB structure with no cavities.

FIG. 11 is a table that sets forth dimensions and parameters A-F. Thediagrams of FIGS. 7-10 are not necessarily to scale, and are notnecessarily accurate in terms of the number and size of the cavities,but rather are provided for illustrative purposes. The dimensions andparameters A-F set forth in FIG. 11 are controlling. The values B-D areselected to cover a determined range of realistic and workable cavitiesand cavity distributions, for a range of realistic and workable bottommetal plate thicknesses. In a preferred embodiment, for thermalconduction reasons, the cavities should not be so deep that they reachthe central ceramic sheet member portion of the DMB. The parameter F wasdetermined by considering an applied typical thickness of thermal greaseof about 50-60 micrometers, where this thickness of thermal grease asinitially applied covers the A by A square area. Not all of this thermalgrease is then later to be consumed into the aggregate cavity volume asthe module operates, so the maximum aggregate cavity volume isdetermined accordingly. The parameter F was set to be somewhat abovethis aggregate value. The parameter E was determined by considering theminimum number of cavities B for the range of realistic examples, andthe minimum volume of each cavity for the range of realistic examples,and by multiplying the two numbers to obtain a product, and thenselecting the parameter E to be slightly below the product. Thestructures defined by the parameters A-F are not to include or toencompass some metal surface that just incidentally happens to have acavity or cavities, but that otherwise has nothing to do with DMBsubstrates for carrying power semiconductor devices. Parameters A-F areselected to exclude such incidental structures.

When the DMB structure 31 is attached and pressed to the metal baseplate2 as explained above, some of the thermal grease will be forced up intothe cavities. Some air that was in the cavity may remain in the cavitybut may be somewhat compressed. As a result of the attachment of themetal baseplate 2, there can be a smaller final distance between themetal of the bottom metal plate 36 and the metal of the top surface ofthe metal baseplate 2. The top surface of the metal baseplate 2 may notbe perfectly flat. Likewise, the bottom surface of bottom metal plate 36may be somewhat irregular, especially when the upper part of the moduleis forced against the metal baseplate 2 with substantial force.Accordingly, there may be areas of relatively good metal-to-metalcontact, and other areas of more separation between the metal of thebottom metal plate 36 and the metal of the top of the metal baseplate 2.The average overall separation, however, is smaller as compared to amodule in which the bottom surface of the DMB is planar and has nocavities.

The cavities in the bottom metal plate 36 can be thought of as workingmuch in the same way as the grooves in a car tire. A car tire hasgrooves. As the tire is pressed against water-covered pavement, some ofthe water is forced out from under the tire and up into the grooves.This results less water being disposed under the tire between the tireand pavement. This increases the amount of rubber that is actually incontact with the pavement as the tire rolls over the pavement. Insomewhat the same way, in the example of the cavities in the bottommetal plate 36, some of the thermal grease is forced up into thecavities as the DMB structure 31 and the metal baseplate 2 are pulledtogether (under the force of the four screws 22-25). This increases theamount of metal of the bottom metal plate 36 that is actually in contactwith metal of the top of the metal baseplate 2.

In addition, there is slightly more metal surface area on the bottom ofthe bottom metal plate 36 as compared to the same DMB structure if itwere to have no cavities. In some small sense, the bottom of the bottommetal plate 36 is slightly finned. In this sense, the cavities in thebottom metal plate 36 can be thought of as working like the fins on aheatsink. Fins are commonly added to a heatsink in order to increasethermal transfer (to reduce thermal resistance) between the highertemperature heatsink material and a lower temperature fluid medium (suchas air) into which the heat is to be discharged. In somewhat the sameway, in the example of the cavities in the bottom metal plate 36, themetal of the bottom metal plate 36 can be thought of as the finnedheatsink and the thermal grease can be through of as the lowertemperature medium into which heat from the heatsink is to bedischarged. Thermal transfer from DMB structure to thermal grease may beimproved by making the bottom of the metal base plate 36 more finned andby increasing the surface area of the bottom surface of the metal baseplate 36.

The bottom surface that is referred to above as being slightly finnedeither has, or is believed to have, the following two advantages. First,by the consumption of thermal grease the fin-like structures work likeshort circuit paths for heat flow. The copper structures serve asthermal bridges through an insulating layer, because the thermal greaseor phase change material or solder are, from a thermal management pointof view, relatively insulating layers compared to the higher thermalconductivity of copper. Second, the thermal grease or phase changematerial has a tendency to dry out at higher temperatures (>100 degreesCelsius). If this layer were to dry out, then thermal conductivitybetween the finned surface and the underlying heat-absorbing surfacewould degrade. The cavities store an amount of thermal grease or phasechange material, and this has the effect of slowing down the dry outprocess.

In a second embodiment, there is a phase change material (PCM) betweenthe DMB structure 31 and baseplate 2 as opposed to thermal grease. ThisPCM material may be applied as a paste, at room temperature, to thebottom surface of the bottom metal plate 36. The PCM paste may beapplied in a pattern so that it covers some parts of the bottom surfaceof the DMB structure 31 but does not cover others. After being appliedas a paste at room temperature, the PCM paste is cured under an elevatedtemperature so that the PCM develops a rubber-like composition. Theupper part of the module housing, with the DMB structure 31 having itslayer of cured PCM, is then attached to the metal baseplate 2 asexplained above. The four screws 22-25 are tightened to hold the metalbaseplate 2 in place, and to keep the metal baseplate 2 pressing againstthe bottom of the DMB structure 31 with a desired pressure. When themodule 1 is then used in an electronic circuit or application, theelectronic components mounted to the top of the DMB operate and generateheat. This heat causes the temperature of the DMB substrate 31 toincrease. At about 45 to 60 degrees Celsius, the PCM undergoes a phasechange and becomes a lower-viscosity, cream-like material. This materialwets the interface between metal plate 36 and baseplate 2. Some of thePCM flows up into the cavities as explained above, and this improves thethermal transfer between the DMB structure 31 and the metal baseplate 2as explained above.

FIG. 12 is view of the detail area 53 of FIG. 8, but in a thirdembodiment in which the cavities do not extend all the way through thebottom metal plate 36.

FIG. 13 is a cross-sectional side view of the detail area 53 of FIG. 12.

FIG. 14 is a diagram of the bottom of the DMB structure in accordancewith a fourth embodiment. In this embodiment, the cavities are notcircular from the top-down perspective but rather are elongatedstrip-like trench structures. Even though the strip-like cavities havean elongated dimension, they still have the same minimum inside width Cas do the cavities of FIG. 7. The strip-like trench cavities can appearas FIG. 10 in cross-section, or can appear as FIG. 13 in cross-section.For purposes of counting cavities, such an elongated strip-like cavityis considered to be within the “A by A square area” if a part of it isin the area, and another part is outside.

Although a few illustrative examples of cavity shapes are set forth inthe description above, it is to be understood that there are manydifferent cavity shapes that can be imagined. For example, the followinglayout of cavities can satisfy the requirements of FIG. 11: a pattern offirst narrow trenches that extend parallel to one another in a firstdirection, with a pattern of second narrow trenches that extend parallelto one another in a second direction, where the first and seconddirections are perpendicular to one another, and where the first andsecond narrow trenches cross each other in a grid-like pattern. Inanother example, a matrix of a plurality of cross-shaped trenches cansatisfy the requirements of FIG. 11. A DMB structure having any of thesedifferent cavity shapes is encompassed within the novel aspect disclosedin this patent document provided that the requirements set forth in thetable of FIG. 12 are satisfied.

Although the DMB structure 31 having the cavities is described abovewith either thermal grease or a phase change material, the DMB structure31 with the cavities can also be soldered to another structure. Forexample, the DMB structure 31 can be soldered to the metal baseplate 2.In one example, solder paste is placed between the DMB structure 31 andthe metal baseplate 2. In another example, solder foil is placed betweenthe DMB structure 31 and the metal baseplate 2. The stack structure isthen heated in a vacuum oven to about 350 degrees Celsius such that thesolder becomes liquid. A partial vacuum (pressure below atmosphericpressure) is then applied to the oven, and this causes air to be removedfrom the over and the structure being soldered. Some of all of the airin the cavities may, for example, be removed due to the use of thispartial vacuum. The stack is then allowed to cool so that the soldersolidifies. After this process, dice can be soldered to the top of theDMB structure. Alternatively, the dice can be soldered to the top of theDMB structure in the vacuum oven heating step. Wire bonding is thenperformed to connect dice and/or islands together on the top of the DMBas desired. Next, a first plastic module housing portion is placed downover the DMB structure. This first plastic module housing portion may beglued to the metal baseplate. Large external connection terminals arethen slid down into accommodating grooves in the first plastic modulehousing portion. Wires are attached to couple the electronic componentsand the islands on the top of the DMB structure to the smaller pin-likeexternal connection terminals. An amount of the silicone gel is pouredover the electronics in the module housing. After the silicone gel hascured, then the second and third plastic module housing portions 4 and 5are attached. The plastic module housing portion 4 may be glued to thefirst plastic module housing portion 3. After being glued, it may alsobe held in place by pins 6 and 7. Accordingly, rather than thermalgrease or a phase change material being in contact with the bottomsurface of the DMB structure having the cavities, in some applicationssolder is in contact with the bottom surface of the DMB structure havingthe cavities. This solder can either attach the DMB structure to anotherpart of the module, or the solder can attach the module to anotherobject such as a heatsink, or a part of the printed circuit board.

FIG. 15 is a perspective view of the bottom of the metal baseplate inaccordance with a fifth embodiment. In this embodiment, the metal sheetthat has the novel cavities is not the bottom metal sheet of the DMBstructure, but rather is the bottom of the metal baseplate 2. The bottomsurface of the metal baseplate 2 is provided with a two-dimension matrixof the cavities so that the requirements of the table of FIG. 12 aresatisfied. More generally, the metal surface of any semiconductorpackage that is to contact a flat metal heatsink surface when thesemiconductor package is mounted can be made to have cavities asdescribed above. The cavities described above can be provided onsemiconductor device packages other than power semiconductor devicemodules.

Although certain specific embodiments are described above forinstructional purposes, the teachings of this patent document havegeneral applicability and are not limited to the specific embodimentsdescribed above. Although examples are set forth above in which thedistribution of cavities across a planar metal surface is uniform, thedistribution of cavities across a planar metal surface need not beuniform but rather can vary and can be non-uniform. Also, the shape ofthe cavities in a surface need not all be the same. Some cavities may beof one shape, whereas other cavities may be of another shape. The depthof the various cavities in a surface need not all be the same. Somecavities can be deeper than others. Accordingly, various modifications,adaptations, and combinations of various features of the describedembodiments can be practiced without departing from the scope of theinvention as set forth in the claims.

What is claimed is:
 1. A power semiconductor device module comprising: asemiconductor device die; and a Direct Metal Bonded (DMB) substratecomprising: an insulative ceramic sheet member; a top metal plate thatis directly bonded to a top surface of the insulative ceramic sheetmember, wherein the semiconductor device die is attached to the topmetal plate; and a bottom metal plate that is directly bonded to abottom surface of the insulative ceramic sheet member; and a metalbaseplate having a substantially planar metal surface, wherein thesubstantially planar metal surface forms an outside surface of the powersemiconductor device module, wherein the metal baseplate has a pluralityof cavities each of which extends into the metal baseplate from thesubstantially planar metal surface, wherein the metal baseplate is inthermal contact with the bottom metal plate.
 2. The power semiconductordevice module of claim 1, further comprising: an insulative housing;wherein the bottom metal plate of the DMB and the metal baseplate aredisposed so that an amount of thermal grease is in contact with both atop planar surface of the metal baseplate and the bottom surface of thebottom metal plate.
 3. The power semiconductor device module of claim 1,further comprising: an insulative housing; wherein the bottom metalplate of the DMB and the metal baseplate are disposed so that an amountof a phase change material is in contact with both a top planar surfaceof the metal baseplate and the bottom surface of the bottom metal plate.4. The power semiconductor device module of claim 1, further comprising:an insulative housing; wherein the bottom metal plate of the DMB and themetal baseplate are disposed so that an amount of solder is in contactwith both a top planar surface of the metal baseplate and the bottomsurface of the bottom metal plate.
 5. The power semiconductor devicemodule of claim 1, wherein an A by A square area of the metal baseplateincludes at least a part of each of B cavities, wherein each of the Bcavities of the A by A square area has a minimum inside width dimensionthat is not more than C millimeters across, wherein each of the Bcavities has a depth that is less than D millimeters; wherein A is 10.0millimeters, wherein B is ten, wherein C is 1.0 millimeters, wherein Dis 0.5 millimeters.
 6. The power semiconductor device module of claim 5,wherein each of the B cavities is a circular recess into thesubstantially planar metal surface.
 7. The power semiconductor devicemodule of claim 5, wherein each of the B cavities is an elongatedstrip-like trench in the substantially planar metal surface.